1. Field of the Invention
The present invention relates to a method for reducing nitrogen oxides from exhaust gases which are discharged from internal combustion engines, burners, nitric acid production plants, etc.
2. Description of the Related Art
The exhaust gases which are discharged from internal combustion engines of automobiles, burners, nitric acid production plants, etc., contain acidic components such as nitrogen oxides (NOx), and are said to be the main cause of air pollution. Accordingly, methods for reducing nitrogen oxides are studied from various standpoints.
Conventional methods of reducing nitrogen oxides include various processes using catalysts. Among such processes, a process comprises adsorbing nitrogen oxides with a catalyst and simultaneously decomposing the adsorbed nitrogen oxides into nitrogen and oxygen, and then allowing the thus obtained oxygen to react with reducing substances such as carbon monoxide, hydrogen, and ammonium gas being incorporated in the atmosphere. In short, the process comprises reducing nitrogen oxides by using those reducing substances. The catalysts used in the processes are those comprising noble metals such as palladium, platinum, and rhodium, being carried on a porous carrier such as of alumina.
However, the aforementioned reducing methods suffer from a decrease in nitrogen oxide reduction under an oxidizing atmosphere; i.e., under atmospheric conditions in which no reducing substances such as carbon monoxide, hydrogen, and ammonia gas are present, or in which an excess amount of oxygen is incorporated. Under such atmospheric conditions of low concentration of reducing substances, the oxygen having been produced by decomposition of nitrogen oxides remains without being consumed, and even if a reducing substance may be present, the substance reacts preferentially with oxygen being present in excess, which prevents effective reduction of nitrogen oxides.
In exhaust gases from an automobile, for example, the amount of oxygen increases with the increase in the ratio of air to fuel (air-fuel ratio) and it exceeds the amount of oxygen necessary for complete combustion of the unburned components. Similarly to the aforementioned state of excess oxygen, nitrogen oxides can no longer be reduced under such a state.
In the light of such circumstances, one of the present inventors has studied a method for efficiently reducing nitrogen oxides in an oxidizing atmosphere to establish a method as disclosed in Japanese Laid-Open Patent Publication No. 190830/1992. The method comprises bringing a gas to be treated containing water vapor and nitrogen oxides into contact with a proton conductive solid electrolyte having an anodic portion and a cathodic portion on the surface thereof. According to this method, the proton is generated on the anodic portion by electrolytic oxidation of water vapor, while the proton thus generated is moved to the cathodic portion to be evolved thereon as hydrogen. The resulting hydrogen is used for reducing and removing the nitrogen oxides being incorporated in the gas to be treated. The cathodic portion contains reducing catalysts such as CuO, CoO, and Cu-ion exchanged zeolites to accelerate reduction of nitrogen oxides.
Although the method above enables reduction of nitrogen oxides even under an oxidizing atmosphere, the present day concern on environmental problems such as of exhaust gases demands a further increase in efficiency for reducing nitrogen oxides in an oxidizing atmosphere.